RHESSys and SWAT simulations for modeling of long term water yield GEOG711 December 6, 2007 Yuri Kim Objectives Understanding RHESSys and SWAT Try RHESSys simulation with 1) only hydrologic calibration 2) including vegetation calibration Comparing two SWAT simulation results Capturing Long term water yield trend in land cover conversion area by model simulation Simulation setting Simulation area: Flat river watershed Precipitation data: averaged precipitation data with three stations Three time series 1) Daily scale: Calibration and Validation 2) Monthly scale: 1926 ~ ) Annual scale: 1926 ~ 2005 Streamflow residual analysis Q Residual = Observed Simulated stream discharge value Results 1 : RHESSys Simulation Parameter Sensitivity m mutiplier is the most sensitive parameter (testing parameters: Ksat, m, mv, Ksatv, and gsmax) Calibration (1998~1999) w/o growthwith growth CalibrationValidationCalibrationValidation NS LogNS Validation (1996~1997) w/o growthwith growth CalibrationValidationCalibrationValidation NS LogNS Calibration and Validation results Calibration resultNS and LogNS valuesof two simulations are similar. Validation w/o growth simulation show too low LogNS value. Therefore, with growth result is better than w/o growth one in terms of lowflow. However, validation NS value of with growth simulation is too low, Therefore, RHESSys simulation needs more calibration. Monthly scale (1926~2005) Red circle: rainfall related Yellow circle: non-rainfall related Monthly scale (1926~2005) (continue) Model overestimated streamflow during low rainfall events. Model underestimated streamflow during high rainfall events. RHESSys month by month result analysis February, August, and December show decreasing trend though R2 is not that high. High Q residual means large difference between observed and simulated value. This can be acceptable in earlier period of the simulation because this difference can be caused by the LULC change. However, the problem is that Q residual value also high at some point during calibration and validation period. Again, RHESSys needs more calibration. Annual scale Two simulation show similar pattern and value. Result 2 : SWAT Simulation Parameter Sensitivity Three parameters show sensitivity with LogNS (i.e. low flow) out of 15 parameters. Calibration #3907 (1998~1999) #3907#1325 CalibrationValidationCalibrationValidation NS LogNS Plot with maximum (95%) and minimum (5%) boundary of SWAT simulation (Threshold=LongNS 0.5) Calibration #1325 (1998~1999) #3907#1325 CalibrationValidationCalibrationValidation NS LogNS Two simulation have same LogNS value, whereas #3907 is better fit in terms of high stream flow. Validation result of #3907 (1996~1997) #3907#1325 CalibrationValidationCalibrationValidation NS LogNS Validation result of #1325 (1996~1997) #3907#1325 CalibrationValidationCalibrationValidation NS LogNS #1325 has higher NS value than that of #3907 but lower LogNS value than that of #3907 in validation process #1325 validation NS value is the highest value of all NS values. Monthly scale 1 (with #3907) Red circle: rainfall related Yellow circle: non-rainfall related Jan., Feb., Mar., and Apr. trend line: Observed > Simulated Monthly scale1 (with #3907) (continue) Oct. trend line: Observed < Simulated Monthly scale 2 (with #1325) Feb., Mar., and Apr. trend line: Observed > Simulated Monthly scale2 (with #1325) (continue) Aug., Sep., Oct., and Nov. trend line: Observed < Simulated Monthly scale #3907 and # 1325 comparison Calibration and validation period of Q residual value tend to be minus; simulation value is higher than observed value. This phenomena is more prominent with #1325 than #3907. #3907#1325 Monthly scale #3907 and # 1325 comparison (continue) Similar trend in both simulations but a little bit more overestimated simulation value with #1325 during calibration period. #3907 #1325 Annual scale (1926~2005) According to the trend line, #3907 produce more Q residual than #1325. #1325 has higher NS value than that of #3907. Also #1325 simulation overestimate stream flow during calibration and validation period. Discussion : RHESSys & SWAT Monthly simulation comparison RHESSysSWAT #3907 Annual simulation comparison Water yield difference: RHESSys > SWAT Conclusions Both RHESSys and SWAT can capture annual long term water yield trend in land use/land cover conversion area. However, the trend lines change sensitively with each currently calibrated parameter set. By current calibration and validation results, RHESSys needs more calibration than SWAT. According to the month by month analysis, there are exceptional big residual difference during calibration and validation period. Model did not appropriately respond to some extreme rainfall events. Joint analysis with precipitation pattern is also required in order to improve modeling process.